In this paper I advance a constitutive argument for the authority of rational norms. Because accountability to reasons is constitutive of rational agency and rational norms are implicit in reasons for action and belief, the justification of rational norms is of a piece with the practice of reasoning. Peter Railton has objected that the constitutive view fails to defend the categorical authority of reason over agents. I respond to his objections, arguing that they presuppose a foundationalist conception of justification that demands a ground for reason's authority outside of the practice of reasoning